Graphite anode treatment

ABSTRACT

An iron impregnated graphite electrode is prepared by treating graphite with a liquid metal such as iron or a mixture of iron with carbon and/or silicon.

United States Patent [72] lnventors Morris P. Grotheer [54] GRAPHITE ANODE TREATMENT 5 Claims, No Drawings [52] US. Cl 1 17/228,

117/114, 136/122, 204/290, 204/294 [Sl] Int. Cl B0lk 3/08 501 Fieldofsearchu, 117 114, 228, 118; 136/120, 121, 122; 204/290, 292, 294

[56] References Cited FOREIGN PATENTS 958,376 5/1964 01661131161111 il7/CB 669,472 8/1963 Canada ll7/CB Primary Examiner-Ralph S. Kendall Attorneys-Peter F. Casella, Donald C. Studley, Richard P.

Mueller, James F. Mudd and William .I. Schramm ABSTRACT: An iron impregnated graphite electrode is prepared by treating graphite with a liquid metal such as iron or a mixture of iron with carbon and/or silicon.

GRAPHITE ANODE TREATMENT BACKGROUND OF THE INVENTION Graphite anodes have been conventionally employed in electrolytic processes for the electrolysis of aqueous solutions of alkali metal halides, both in the production of halogens and caustic as well as alkali metal halates.

The cost of graphite is a large factor in the overall price of electrolytic cell products. Graphite presents a relatively larger cost factor in chlorate production than in chlor-alkali production. conventionally, to improve electrode life the electrode graphite is impregnated with a drying oil which tends to prevent wetting on the interior of the graphite by an aqueous electrolyte.

It was found in applications Ser. No. 789,008, ID 225 l filed of even date herewith by .l. E. Currey, M. P. Grotheer and E. H. Cook, Jr. that, an electrode material comprising a massive graphite structure impregnated with iron exhibited excellent structural and electrical properties. The electrodes contain by weight, based upon the final product, between 0.05-3, percent iron, and preferably, 0.052.5 percent iron in the graphite matrix. More than 3 percent iron does not appear to provide any advantage.

BRIEF SUMMARY OF THE INVENTION In accordance with the instant invention, there is provided a method for preparing iron impregnated graphite which comprises contacting a massive graphite structure with a liquid metal selected form the group consisting of iron and iron mixed with at least one member selected from the group consisting of carbon and silicon, separating said massive graphite structure from said liquid metal and cooling said massive graphite in a substantially inert atmosphere.

DETAILED DESCRIPTION OF THE INVENTION The graphite which is employed in the instant invention may be porous fuel cell grade graphite or standard anode grade graphite. Exemplary of the graphite matrixes under consideration are porous graphite having an apparent density of 0.936 and approximately 58.4 percent voids by volume; porous graphite of apparent density 1.04 having approximately 53.8 percent voids by volume and anode grade graphite of apparent density 1.67 having approximately 25.6 percent voids by volume. These calculated voids are based upon a standard theoretical specific gravity for graphite of 2.25. The preferred graphite for the electrolytic purposes of this invention is preferably that of apparent density between 1.40 and 1.80.

Iron and iron alloys of carbon and silicon may be employed in the practice of the instant invention. Acceptable alloys of iron which do not otherwise interfere in the use of iron impregnated graphite as an electrode in chlor-alkali and chlorate production include those Fe-C alloys approximately in the range of 94-999 Fe and 0.1-6 C; while the applicable silicon containing alloys include those with a silicon content as high as percent by weight. For example, grey cast iron contains 94 Fe, 3.5 C, 2.5 Si while the high silicon content cast irons contain approximately 84.3 Fe, 14.5 Si, 0.85 C and trace amounts of Mn, P and S.

The amount of iron deposited within the pores of a graphite electrode matrix may be controlled by limiting the exposure time to the liquid metal. Thus, through a rapid dipping technique in liquid metals of hot graphitized carbon prepared essentially in the manner presented in Ind. Eng. Chem. vol. 46, No. 1, pp. 2-11 (1954), when modified by a continuous tube furnace for graphitization in lieu of the described batch type electric graphitization fumace, the graphite surface is rapidly wet with metal which will continue to penetrate into the internal region of the graphite after withdrawal from the molten bath.

The molten iron may be efficiently prepared either by melting iron or an iron alloy through application of heat or by chemical reaction such as may be exemplified by the process 8 Al 3 Fe O 9Fe (molten) 4Al O When a chemical reaction is employed to produce molten iron, the graphite structure to be treated may be placed in a crucible and covered by the reactants. Uponinitiation of the reaction, a pool of molten iron forms at the bottom of the crucible and surrounds the graphite structure effectively penetrating into the pores of the graphite. Removal of the graphite structure while the iron is still in liquid state affords an acceptably impregnated graphite structure.

The process of this invention avoids the-multistep technique presented in copending Application Ser. No. 2251 referred to supra, and provides a direct approach, adaptable to commercial graphite production techniques, for iron impregnation of graphite.

The resulting iron impregnated graphite may be optionally post impregnated with a conventional drying oil such as linseed oil, or it may be used directly as an anode material. It is preferred to seal the iron impregnated graphite with oil to prevent excessive attack by the corrosive contents of the electrolytic cell.

EXAMPLE 1 A block of graphite, lXl'X'r inches was packed in a mixture of iron filings and carbon powder in a fire clay crucible. The mixture was 94 percent by weight iron and 6 percent by weight carbon. The crucible and contents were heated at a temperature between 1,000 to about 1,400 C. over a period of 4 hours and then maintained at 1,400 C. for 1% hours. The crucible was removed from the furnace and molten metal phase was poured out. The graphite block was recovered and allowed to cool in an inert nitrogenous atmosphere. When cool, some large metallic beads were removed from the graphite surface by gentle scraping. The product was dark gray in color and exhibited some pitting. Under 10 power magnification, very small metallic beads were observed on the graphite surface.

The resulting graphite block was exposed as an anode to a solution containing about 300 grams per liter sodium chloride. The electrode chlorine overvoltage was 1.27 volts at one ampere per square inch whereas the chlorine overvoltage for conventional chlor-alkali grade, oil impregnated graphite exhibits approximately 1.35 volts at l ampere per square inch.

EXAMPLE 2 A graphite electrode is manufactured by preparation weighing, and mixing of the raw materials; (normally high purity extrusion carbon and a binder such as coal tar pitch) of the heated mixture in the desired shape and cross section; baking in a gas-fired furnace and graphitization in a continuous tube electric furnace. The electric furnace is operated at a temperature sufficient to produce a graphitization temperature of bout 2,800 C. Upon removal from the furnace, the graphite electrodes are quenched in a bath of molten iron which is maintained at a temperature of about 1,400 C. An intermediate cooling step to prevent fracturing of the graphite may be employed if necessary. The graphite is impregnated with iron from the molten bath, withdrawn and cooled slowly to room temperature. An inert atmosphere is provided for the graphite as it cools to about 500 C., either in the form of an inert gas such as CO, argon or nitrogen, or via surface oxidation of the graphite in a limited atmosphere (withthe production of CO).

Elemental analysis of an iron impregnated electrode prepared in a manner similar to the procedure of example 2 demonstrated that 0.15 percent iron was present in the graphite structure. lmpregnation was evidenced throughout the sample.

The anodic polarization studies of the iron impregnated graphite were performed by masking with liquid latex rubber all but 1 square inch of exposed electrode surface. The polarization measurements were made in a stainless steel box cell. A

Teflon cloth diaphragm separated the anolyte from catholyte. 5

The electrolyte was 295 grams per liter sodium chloride at C. An Anotrol Model 4100 potentiostat in conjunction with a Houston X-Y recorder was used to obtain the anodic polarization. Compared to conventionally employed graphite electrode material, the iron impregnated graphite electrode prepared in accordance with the instant invention demonstrated an anode potential with reference to a saturated calomel electrode about 0.1 volt below that of a conventional graphite electrode at l ampere per square inch.

in operation, the electrode prepared by the process of this invention exhibits reduced chlorine overvoltage in comparison to conventional chlor-alkali and chlorate grade graphite electrodes when employed in chlor-alkali and chlorate production. Furthermore, the electrode of this case is considerably less prone to consumption in chlorate production than in a conventional chlorate grade graphite anode.

Likewise, the increase in cell voltage with decrease in NaCl concentration is considerably less with the iron impregnated graphite electrode prepared in accordance with the procedures of this invention than it is with a conventionally oil treated graphite electrode. For example, during a comparative experiment employing a normal chlorate graphite electrode with an iron impregnated chlorate graphite electrode, as the concentration of NaCl decreased from about 250 grams per liter to about 150 grams per liter, the cell voltage increased from about 3.25 volts to 3.6 volts with the normal graphite while with iron impregnated graphite, the increase in cell voltage was from about 3.15 to 3.35 volts.

Although it is believed that the iron deposits in the iron impregnated graphite of this invention is singularly found in the pores of the graphite matrix, it is applicants desire not to be bound by that theory because a portion of the iron may actually be in an intercalated state. Hence, it is desired to cover the invention in any of its operative forms as iron impregnated graphite whether that iron appears in intercalated form or as heterogeneous deposits within the pores of the graphite matrix.

We claim:

1. A process for the production of an iron impregnated graphite electrode which comprises heating a formed carbonaceous material to produce graphite having an apparent density of between 1.40 and 1.80 and quenching said graphite in a liquid metal selected from the group consisting of iron, an iron carbon mixture containing about 0.1 to 6 percent carbon by weight, and an iron silicon mixture containing below about 15 percent silicon by weight.

2. The process of claim 1 in which the quenching step is sufficient to impregnate said graphite with iron in an amount between about 0.5 to about 3 percent by weight.

3. The process of claim 1 in which said liquid metal is an iron-carbon mixture containing from about 0.1 to 6 percent carbon by weight.

4. The process of claim 1 in which said liquid metal is an iron-carbon mixture containing between 3 to 4 percent carbon by weight.

5. The process of claim 1 in which said liquid metal contains silicon in an amount below about 15 percent by weight.

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2. The process of claim 1 in which the quenching step is sufficient to impregnate said graphite with iron in an amount between about 0.5 to about 3 percent by weight.
 3. The process of claim 1 in which said liquid metal is an iron-carbon mixture containing from about 0.1 to 6 percent carbon by weight.
 4. The procesS of claim 1 in which said liquid metal is an iron-carbon mixture containing between 3 to 4 percent carbon by weight.
 5. The process of claim 1 in which said liquid metal contains silicon in an amount below about 15 percent by weight. 